Critical to the industrial use of carboxylic acids which are generated by fermentation of carbohydrate-containing substrates using various microorganisms is the cost-effectiveness and efficiency of removing and cleaning up the lactic acid from these aqueous fermentation solutions, which contain not only the carboxylic acid or the carboxylic acid salts but also further organic acids, other fermentation byproducts, microorganisms and the constituents thereof and also remnants of the substrates, such as sugars. These impurities interfere with the subsequent further processing of the carboxylic acids generated. For example, lactic acid is polymerized to form polylactic acid in order to produce biogradable plastics. For this purpose, it is necessary to use extremely pure monomer in order to achieve a high degree of polymerization of the lactic acid. This has been known for a long time and is disclosed by, for example, J. Dahlmann et al., British Polymer Journal, vol. 23 (1990), pp. 235-240.
A similar situation is known to exist for succinic acid for example. The grades of the succinic acid generated can be differentiated by subdivision into a technical grade having a succinic acid content of at least 97% by mass and a succinic acid which is especially suitable for use for polymerization (polymer grade or 1,4-butanediol grade) and has a content of at least 99.5% by mass.
A multiplicity of patents provide a description of obtaining succinic acid from fermentation solutions, including                extractive processes using extraction agents such as tributylamines, trialkylamines, olefins, various alcohols and aromatic hydrocarbons,        processes using calcium hydroxide and sulfuric acid, producing gypsum as byproduct,        processes using electrodialysis,        thermal methods such as fractional distillation or thermally graduated chromatography,        high-pressure extraction using CO2,        membrane methods such as, for example, reverse osmosis and other filtration processesand these patents also discuss interlinkings of these methods and modification by further steps corresponding to the prior art. Such methods are described, inter alia, in patent documents DE 69821951 T2; DE 69015233 T2; DE 69015019 T2; DE 69006555 T2; DE 69015019; DE 60028958T2; DE 10 2004 026152 A1.        
In addition, a multiplicity of methods concerning the purification of lactic acid are known.
For example, some patents teach the use of distillation to purify lactic acid from aqueous solutions. EP 0986532 B2 takes advantage of such a method. DE 10 2007 045 701 B3 discloses combined extraction with linear n-trioctylamine (TOA) and distillation. Further possibilities known in the literature are electrodialysis or esterification with an alcohol, with distillation and then hydrolysis of the ester formed likewise being carried out thereafter. These methods are extremely cost-intensive. Moreover, distillation has the disadvantage that a portion of the carbohydrates is also always coextracted, leading to a deterioration of the yield of the entire process and impeding the isolation of the product.
Processes using calcium hydroxide and sulfuric acid, with gypsum being produced in large quantities as byproduct, are also known. In this connection, it was additionally found that lactic acid can be isolated using chromatographic methods from, for example, a sulfuric acid-acidified fermentation broth containing not only free lactic acid but also ammonium and sulfate ions. For example, DE 69815369 T2 describes, inter alia, the removal of lactic acid from aqueous mixtures by adsorption to a solid adsorbent; in this case, preference is given to using a solids adsorbent which adsorbs lactic acid as opposed to lactate. More particularly, according to the above document, weak anion exchangers are a possibility for the isolation of lactic acid. DE 10 2009 019 248 A1 further describes chromatographic methods for purifying organic acids, in particular lactic acid, by carrying out simulated moving bed chromatography.
WO 2006/124633 A1 describes a process for producing ammonium lactate by fermentation. During the fermentation, the ammonium salt of lactic acid is formed, which salt can be removed from the fermentation solution by, for example, extraction. In a subsequent step, the ammonium salt can be split very easily using weak acids or carbon dioxide. In said step, the free lactic acid is obtained, which can then be cleaned up by, for example, distillation.
WO99/19290 describes fermentation of lactic acid with subsequent filtration and extraction, it being possible for the extraction to be an adsorption. This document does not disclose the nature of the interaction with the adsorption solid phase. A similar method is disclosed in WO93/06226, the adsorption solid phase in this case being provided with tertiary amino groups and the rate of production of free acid being increased as a result. EP0135728 also teaches the isolation of enzymatically generated carboxylic acids via adsorbers provided with tertiary amino groups. In this case, the fermentation takes place via column-immobilized cells.
A disadvantage of many methods is that additional substances are supplied to the process, which substances must no longer be present in the target product or the traces of which substances in the target product may lead to limitations in the quality and the applicability of the product. The practical implementation of the methods is also associated in some cases with considerable technical complexity and considerable energy consumption.
For instance, DE19939630C2 discloses a method for finely cleaning up aqueous solutions containing organic acid produced by fermentation, such as, for example, citric acid, lactic acid, succinic acid or tartaric acid. In said method, the solution coming from the fermentation is first filtered and the resulting solution is subsequently delivered across an anion exchanger and a subsequent adsorber resin bed. The adsorber resin bed used for the fine cleanup is initially loaded with OH− ions and binds the product acid. Moreover, further purification of the filtered solution, downstream of the filtration, in order to remove impurities present such as, for example, cell debris, carbohydrates, nutrients, amino acids and sugars is not mentioned.
DE69815369T2 describes the isolation of lactic acid from a fermentation broth by adsorption of the lactic acid to a solid adsorbent with subsequent generation of the lactic acid from the solid adsorbent, with removal of the biomass taking place beforehand by filtration. In this case, the solid adsorbent can be a poly-4-vinylpyridine resin or a tertiary polystyrene divinylbenzene amine resin. An intermediate step for removing further impurities is not taught here either.